This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to a scanning machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. Thermal-inkjet printers and processes are of greatest interest.
The invention is applicable, however, in other types of units such as, for example only, piezodriven inkjet printers and hot-wax transfer printers. The invention coordinates dither masking and printmode techniques in such a way as to optimize image quality with minimal disturbance of preestablished data structures and control programming.
This section introduces the basis and history of a particularly persistent category of undesirable printing artifacts that impair the quality of images made with incremental printers. Those artifacts are very peculiar-appearing and repetitive patterns that appear superimposed on, usually, rather uniform colorant fields in the middle tonal range.
These patterns are particularly undesirable because they repeat, and therefore often manifest themselves as spurious banding or tiling within the image. (Some curious shapes may appear even in the absence of repetition or other distracting systematic character, but such shapes generally go unnoticed or accepted.)
(a) Renditionxe2x80x94Incremental printers are generally capable of creating only a relatively small number of colors at each picture-element (pixel) positionxe2x80x94particularly as compared with the millions of colors that can be developed on a computer or television screen, or the virtually continuous gradations available through photography. To enable incremental printers to simulate the finer gradations provided by such other technologies, workers in the incremental-printing field have developed techniques known as xe2x80x9crenditionxe2x80x9d.
Prominent rendition methods include scattered dither and error diffusion. Such techniques aim to reproduce midtones of colors by, in effect, averaging colorsxe2x80x94over a number of pixels that is relatively large in comparison with just a single pixel, but still rather small as compared with the spatial resolving power of the human eye.
Scattered dither in printed images thus smoothly spreads fine dots on the printing medium in such a way that the average reflected light per unit of surfacexe2x80x94taking into account the unprinted space on the medium, between dots, as well as the printed dotsxe2x80x94matches the light intensity of the desired tone. It is important to spread the dots smoothly because unevenness in spatial distribution of the dots creates grainy or noisy images that are often objectionable to the viewer. Other methods such as error diffusion can assign locations to dots to be printed without generating repeating patternsxe2x80x94but require much more computing power in the computer or host, or both. Error diffusion, furthermore, although producing no repetitive patterns does often generate systematically propagating patterns. These patterns too can be obtrusive when seen in nominally uniform fields. Error diffusion is therefore most typically reserved for photograph-like pictures or other images having many small details that obscure the propagating patterns.)
Heretofore it has been recognized by many workers in this field that unevenness in dither-type rendition may be associated with particular dither matrices used. Some such efforts are discussed below.
It has also been recognized by many, however, that unevenness may be associated with a separate function known as xe2x80x9cprintmaskingxe2x80x9d. Printmasks are used to allocate or distribute the selected dots among successive printhead passes over the printing medium. These efforts too are outlined below.
(b) Ditherxe2x80x94As to dither matrices, unevenness in spatial distribution of dots may arise from the quality of the matrices themselves, or from the interaction of such dither matrices with other parts of the writing system that affect the final placement of dots on the print medium. More specifically, consistent dot-placement error (DPE) when combined with the use of dither matrices creates unevenness that repeats consistently throughout an image, creating intrusively unpleasant patterning and banding.
For present purposes it is important to understand precisely how such patterns develop so that they physically appear in a finished, printed image on a physical printing medium. The prime mechanism for development of a pattern is the repetitive recurrence of particular dot-placement errors (DPEs) in the same positions of a particular tiled mask.
DPE is basically a fingerprint of each infinitesimal irregularity of firing direction and speed, and drop volume as well, for each of the many different nozzles in a particular printhead. Image dithering is performed using threshold matrices that provide the spatial distribution of dots to be printed for every tonal value.
Now, when a certain DPE characteristic is registered in a particular way with a certain dither mask (or print-mask), and used to print a midtone field, the unique printhead DPE characteristic and unique mask characteristic in combination produce, potentially, a distinctive set of shapes.
The qualifying term xe2x80x9cpotentiallyxe2x80x9d is used here because it has not yet been shown how the dither-mask pattern is preserved through the printing process and expresses itself through the printhead DPE pattern to form noticeable patterns on the printing medium. That will be demonstrated shortly. For the moment, to enable further intermediate discussion, that demonstration may be taken on faith.
Since each matrix has limited size, it must be stepped and repeated, over and over in both directions, to entirely fill all the space to be printed with the tone. Thus if the midtone field is uniform or roughly uniform, throughout an area that extends over multiple units of the same mask, then those distinctive shapes appear tiled across and down that area. The resulting appearance is sometimes quite conspicuous, and in extreme cases even distracts from the subject matter of the image.
Accordingly some previous workers have striven to provide dither masks having an ideal degree of randomness. The objective of such work has been to avoid both the appearance of patterns due to excessive regularity within earlier dither masks, on the one hand, and the appearance of graininess due to excessive randomness on the other handxe2x80x94while at the same time achieving a desired level of vividness in resulting colors. Some advances in this area are due to Qian Lin, Paul Dillinger and Alexander Perumal as reported in their above-mentioned documents.
Such work has been extremely useful and successful in many regards, including virtual elimination of systematic-looking patterns within individual dither masks. These innovations, however, have fallen far short of eliminating larger patterns.
This is because the pattern of an individual mask, even though not itself systematic-looking or even noticeable when considered internally and singly, does look conspicuously systematic when stepped and repeatedxe2x80x94in its entiretyxe2x80x94multiple times across or down a page. The contorted, random crawling patterns (FIG. 1) are sometimes reminiscent of inkblots, gray matter, or, appropriately, a can of worms. They are at least as disturbing as the systematic patterns created with earlier, internally more-regular masks.
The above-mentioned Askeland documents report methods for attacking this repetition of dither-mask/DPE interactions. One of Askeland""s techniques provides plural xe2x80x9csuperpixelsxe2x80x9d (related to dither masks) and introduces randomness into the selection of the superpixel to be applied at each pixel position.
The other Askeland document defines plural, colorimetrically equivalent tonal levels that can be used in randomized selection of either dither masks or printmasks. Askeland displays results that appear to represent significant improvement.
For purposes of the present invention it is noteworthy that the dimensionsxe2x80x94in pixelsxe2x80x94of dither masks are essentially always powers of two. This fact arises from the well-known use of binary devices virtually throughout the computing world.
One consequence of that predominant usage is a very great efficiency and convenience advantage which results, in both data handling and program management, from working with values that are simple in binary terms. Those values are of course powers, or at the very least multiples, of two.
(c) Printmasksxe2x80x94Many other workers have looked to the printmasking function for cures to the twin problems of unevenness and patterning. Thus the earlier-noted work of Doron and of Garcia explores divergent approaches to obtaining randomness in printmasksxe2x80x94and optimizing the degree or intensity of that randomness.
Printmasking is a well-known and important function that promotes two important objectives. First, it tends to avoid the deposition of too much colorant, in excessively localized areas of the printing medium, within too-short time periods. In the absence of such provisions, colorant may coalesce within the pixel grid, causing not only undesired color bleed (mixing) but also far more physical effects such as offset of colorant onto the backs of neighboring printed sheetsxe2x80x94or even cockle (puckering of the printing medium itself), and blocking (sticking to such adjacent pages).
By laying down in each pass of the printhead just a fraction of the total colorant required in each section of the imagexe2x80x94so that any areas left white in each pass can be filled in by one or more later passesxe2x80x94printmasking reduces the amount of liquid that is all on the page at any given time. Results include a reasonable degree of control of bleed, blocking and cockle, and sometimes shorter drying time.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a xe2x80x9cprint modexe2x80x9d. An earlier generation of printmasks unfortunately contained regular internal repetitions, such as square or rectangular checkerboard-like patterns.
Those regular repetitions tended to create objectionable moire effects when frequencies or harmonics generated within the patterns were close to the frequencies or harmonics of interacting subsystems. Hence it is very appropriate that more-recent efforts have focused upon the internal randomization of printmasks.
As the second of its two primary functions mentioned above, printmasking is also exploited in conjunction with plural passes over each region of the printing medium to hide the edges of swaths created by the scanning printheads. To avoid such problems of horizontal banding, a print mode may be constructed so that the medium advances between each initial-swath scan of the pen and the corresponding fill-swath scan or scans.
In fact this can be done in such a way that each pen scan functions in part as an initial-swath scan (for one portion of the printing medium) and in part as a fill-swath scan. This technique tends to distribute rather than accumulate print-mechanism error that is otherwise impossible or expensive to reduce.
The previously mentioned Cleveland patents discuss at length the use of space- and sweep-rotated printmode masks. In these techniques, a printmask is bodily shifted with respect to the printheadxe2x80x94in such a way that the head, in its successive passes across the print medium, addresses subarrays of dots that fit neatly in among one another to eventually account for all the pixels in the image.
Cleveland also points out that operating parameters can be selected in such a way that, in effect, rotation occurs even though the pen pattern is consistent over the whole pen array and is never changed between passes. Figuratively speaking this can be regarded as xe2x80x9cautomaticxe2x80x9d rotation or simply xe2x80x9cautorotationxe2x80x9d.
In addition Cleveland taught that steeply angled printmask patterns of separated lines could be used to mitigate problems of arch-shaped artifacts in the top and bottom zones of a page. In those zones there is no print-medium advance between printhead scans, and the medium is not held in tension effectively.
All of these several techniques, however, suffered from the previously mentioned problems of internal regularity within each mask itself. Recognition of these problems has led to intensive effort toward randomization within the patterns.
Most recently such efforts have grappled with the realization that the more random (as distinguished from homogeneous) a pattern is internally, the more distinctive and even bizarre it may appear when stepped dozens of times across and down a page. As explained above in the context of dither masks, visible patterns develop through repetitive recurrence of particular dot-placement errors (DPEs) registered in the same positions of a particular dither mask. It will be shown below that the means by which that dither-mask pattern xe2x80x9cexpresses itselfxe2x80x9d through the particular DPEs is in fact the printmask.
In a midtone field the unique characteristics of the printhead DPE and the two masks, all considered together, produce unique and sometimes surreal shapesxe2x80x94tiled throughout the subject area. Accordingly, most-recent work has confronted the need to provide randomization over areas much larger than the printmask patterns themselves.
Garcia, in particular, has discovered that the human eye is sensitive to pattern repetition within only a relatively narrow range of spatial frequencies. Garcia accordingly teaches that printmask size need not cover an entire image, to eliminate apparent patterns effectively, but rather need only be wide and tall enough to fall outside that range of sensitivity.
Garcia""s sweeping innovations also teach how to make a printmask essentially as large as desired, and how to introduce into it precisely an ideal degree of randomness. They further explain how to accomplish these several tasks while committing minimal printmask memory to the effort.
Doron, in contrast, applies a relatively modest methodology to randomization, somewhat akin to Askeland""s in the dither-mask regime. Doron evidently provides plural printmasks, each relatively small, for each image.
He prescribes run-time random selection of one mask for use in each printing operationxe2x80x94that is, for each printer pass in a repeating series of, say, four or five passes. The printer selects a different one of the masks for each pass, which is to say that different masks are applied to printing of different image portions.
Doron appears to introduce randomness at two stages of his procedures. First, in the mask-building stage his procedure operates in a three-dimensional space that is a vertical stack of pixel grids, each successive grid or plane representing a higher layer of inkdrops applied in succeeding passes, or by later-arriving nozzles in a single pass.
He randomly selects a series of vertical columns in the stackxe2x80x94e.g., columns that underlie particular pixel positions in the topmost gridxe2x80x94for processing, one column at a time. In processing each column he fills-in printing parameters for the several grids in the stack, observing selection rules that refer to columns which have been previously filled in.
The selection rules appear to result in an essentially deterministic series of selected xe2x80x9cparameterxe2x80x9d numbers down the column, given the columns filled in previously. In this way the random order of column selections controls the final overall three-dimensional array of parameters.
Second, of Doron""s two introductions of randomness, his control program at run time randomly selects one of a number of the randomly generated masks for use in each xe2x80x9cprinting operationxe2x80x9d as mentioned above. The degree of randomness in his pseudorandom system appears to be fixed by these two processes.
Although it uses plural masks in conjunction for printing of each image, his system appears to require relatively modest quantities of data storage. It requires more than just the storage for one mask, but less than would be required for permanent storage of all necessary masks (since his printer builds its own masks in the field), and a small amount in total because each of his masks is rather small.
For its efficacy the system attempts to rely upon continuously shifting among several available masks to break up patterning, but the individual masks are small. Doron""s mask height (i.e. parallel to the print-medium advance direction in the pixel grid) is his pen height, preferably one hundred twenty-eight pixels; however, his most-highly preferred mask width is said to be only thirty-two pixels.
Thus if the number of masks is also rather small, some potential for repetitive patterns breaking through may remain. On the other hand if the number of masks is adequately large and the selection truly randomized, then the system may exhibit adverse effects of highly random maskingxe2x80x94as pointed out and explained in the previously mentioned Garcia document.
Doron does not address the issue of optimizing the degree of randomness in his systems. Nonetheless Doron""s methods appear to represent a significant improvement, and are very promising.
For present purposes it bears mention that the dimensionsxe2x80x94in pixelsxe2x80x94of printmasks are essentially always powers of two. This is no coincidence but rather arises from the pervasive use of binary devices noted earlier in discussion of dither masks.
(d) Other interactionsxe2x80x94The present invention proceeds from recognition of another fact about graininess and patterning. Since that realization is associated with the invention, it will be reserved for the following section of this document.
(e) Conclusionxe2x80x94Although great strides have been made in understanding and managing the above-noted sources of undesired patterning and banding artifacts, at least one such source remains and is introduced below. All of such error sources continue to impede achievement of uniformly excellent incremental printingxe2x80x94at high throughputxe2x80x94on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
The present invention introduces such refinement. Before offering a relatively formal discussion or description of preferred embodiments of the invention, a preliminary orientation will be offered. It should be understood that this informal introduction is not a statement of the invention as such.
As explained above, some recent work has focused on banding and patterning due to stepped internal (though random) patterns of dither masks; and other prior work has focused on banding and patterning due to stepped internal (though random) patterns of printmasks. What evidently has not been addressed heretofore is the interaction between the dither masks and the printmasks.
The present invention focuses upon that interaction. Interaction between the two kinds of masks is important because the sizing of both types is determined for data-handling and program-management efficiency and convenience. These considerations, as pointed out earlier, favor data blocks that are in binary sizesxe2x80x94i.e. blocks whose sizes in bytes are multiples of two, and preferably powers of two.
As a result ordinarily the width 101 (FIG. 2) of the printmask 102 fits an integral number of times across the width 103 of the dither mask 104. More specifically, for the illustrated example, that integral number of times (right-hand view of the diagram) is four. Conversely, the height 105 of the dither mask 104 fits an integral number of timesxe2x80x94namely, two timesxe2x80x94down the height 106 of the printmask 102.
Therefore repetitive effects developing in either mask are also seen in the other mask. As a result, any perceptible pattern that develops in printing withxe2x80x94in particularxe2x80x94a dither mask 104 is simply passed through the system by the printmask 102.
Perceptible patterning can be seen in a simulated midtone printout (FIG. 1) using these masks. As predicted by the right-hand view of FIG. 2, the repetitive units of the pattern are vertically adjacent pairs of the square dither-mask unitsxe2x80x94outlined in a white overlay grid.
More specifically, as noted earlier a directional error in a single nozzle translates into a dot-placement problem for certain pixels of the dither matrix. Initially it is the printmask, not the dither mask, that determines which nozzles will print any specified image dot (or dither-mask pixel)xe2x80x94but the printmasks are consistently aligned with the dither mask.
This is the key to the patterning. Because of the interfitting of the dither masks with the printmasks, any specified dither-mask pixel is always printed by a certain specific nozzlexe2x80x94or at most a small number of such nozzlesxe2x80x94and the resulting dot is therefore always displaced in the same way.
A like analysis applies to other kinds of nozzle idiosyncrasies, including inkdrop size, velocity, special tendency to develop a secondary drop or tail, etc. Each such peculiarity contributes to a unique signature for each nozzle.
The unevenness-generating DPE problem is thus applied consistently and repetitively across the tonal area, producing the patterning described above. Aggravating this is the fact that both dither masks and printmasks are often small, making the repetition periodicity very noticeable to human vision.
It was previously promised to demonstrate just how the dither-mask pattern could express itself through a particular printhead DPE pattern to produce patterns on the print medium. It was also proposed that the means for such expression are in fact the printmask.
Now that the demonstration is complete, the reader can appreciate that it depends upon repeating, consistent registration between the two kinds of masks. Therefore these effects can be broken up to a large extent merely by eliminating the consistency of registration between the two kinds of masks.
This can be accomplished by forcing the two kinds of masks to be of different sizesxe2x80x94or to behave as if they arexe2x80x94as fully explained below. The invention further reveals that this can be achieved without deviating from the preference for multiples of two in data handling and program management.
What is more, the invention teaches that registration between the two masks can be made inconsistent even without disturbing preprogrammed algorithms and hardware, provided only that access to the printmasks (or dither masks) is permitted. Now, with these informal comments in mind, the discussion will turn to a more rigorous presentation of the invention.
In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits.
In preferred embodiments of a first of its facets or aspects, the invention is apparatus for printing a desired image on a printing medium. It operates by construction from individual marks formed in a pixel array.
The apparatus includes some means for establishing a dither mask having a dimensionxe2x80x94i.e., ordinarily either a width or a length. For purposes of breadth and generality in discussing the present invention, these means will be called simply the xe2x80x9cdither-mask establishing meansxe2x80x9d.
Also included in the apparatus are some means for establishing a printmask having a corresponding dimension that is neither a factor nor an integral multiple of the dither-mask dimension. These means, also for generality and breadth, will be called the xe2x80x9cprintmask establishing meansxe2x80x9d.
By the phrase xe2x80x9ccorresponding dimensionxe2x80x9d of the printmask is meant a dimension that corresponds to the previously mentioned dimension of the dither mask. More specifically, if the dimension of the dither mask is a width, then the printmask xe2x80x9ccorresponding dimensionxe2x80x9d also is a widthxe2x80x94or if the dither-mask dimension is a length then the printmask corresponding dimension likewise is a length.
By xe2x80x9cintegral multiple of the dither-mask dimensionxe2x80x9d is meant a value that results from multiplying (1) the dither-mask dimension by (2) a factor that is an integer. If the dither-mask dimension is expressed in terms of pixels, ordinarily that dither-mask dimension too is an integer; hence the resulting product, as well, is an integer.
In addition the apparatus includes some means for using the dither mask to render the image and using the printmask to print the rendered image. These means, again for breadth and generality, will be called the xe2x80x9cusing meansxe2x80x9d.
The foregoing may constitute a description or definition of the first facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention significantly mitigates the difficulties left unresolved in the art.
In particular, the dither mask and printmask of this facet of the invention are distanced from one another in terms of the relationship between their corresponding dimensions. Any resulting patterning that results from the mask interactions can thereby be forced to have a long spatial periodicity.
As explained in a later section of this document, the period is easily controlled, and is readily made much longer than the spatial wavelengths to which the human visual system is sensitive. As a result the patterning, though present in principle, is very inconspicuous (at least to people!) in practice.
Although this aspect of the invention in its broad form thus represents a significant advance in the art, it is preferably practiced in conjunction with certain other features or characteristics that further enhance enjoyment of overall benefits.
For example, as mentioned earlier the dither-mask dimension and printmask dimensions may both be widths, or may be heights. It is more preferable to establish the stated relationships for both widths and heights.
It is also preferred that the dither-mask dimension and the corresponding printmask dimension differ by at least three pixels. (Such a constraint appears helpful to step the patterning away from very fine features of the image itselfxe2x80x94and thereby prevent the appearance of repetition.)
Another preference is that the dither-mask dimension and the corresponding printmask dimension differ by a multiple of two pixels. This constraint is not so much directed to image quality as to maintaining optimum compatibility of the invention with easy programmingxe2x80x94and efficient use of memories and microprocessor operation.
Accordingly it is still more preferable that the dither-mask dimension and its corresponding printmask dimension differ by eight pixels or a multiple of eight pixels. In one particularly advantageous preference following this approach, one of the two mask dimensions is a multiple of 256 pixels and the other differs from that dimension by a multiple of eight pixels.
For multipliers of unity, as an example, if one dimension is 1xc3x97256=256 pixels and the other differs from it by 1xc3x978=8 pixels, then that xe2x80x9cotherxe2x80x9d dimension is 256xc2x18 pixelsxe2x80x94i.e., is either 264 or 248 pixels respectively. The two multipliers need not be the same: for another example using multipliers of four and two, if one dimension is 4xc3x97256=1,024 pixels and the other dimension differs from this by 2xc3x978=16 pixels, then that other dimension is 1,025xc2x116=1,009 or 1,041 pixels.
The apparatus may be one that prints the image in plural colors, the marks being made with respective plural colorants. If so, preferably its several components are in essence replicated for each colorant independently.
Yet another preference, for use with the first major aspect of the invention as defined above, is for use if the apparatus has a scanning printhead that makes multiple passes across the printing medium. According to this preference the printing apparatus includes some means for establishing an offset that is smaller than at least one of said dither-mask and printmask dimensions.
It also includes some means for indexing one of the dither mask and printmask by that offset, between passes of the scanning printhead. The term xe2x80x9cindexingxe2x80x9d in this document refers to stepping or shifting of a mask internally. As will be explained below, this feature provides an alternative way of achieving long periodicities.
This preferencexe2x80x94as a preferred mode of operation of the first major aspect of the inventionxe2x80x94most typically is useful if applied along a dimension orthogonal to the dimension in which the first major aspect of the invention operates. In other words, the xe2x80x9cnonmultiplexe2x80x9d system can be used in one direction, while this alternative xe2x80x9coffsetxe2x80x9d system can be used in a direction at right angles to that one direction.
In preferred aspects of a second of its aspects, the invention is apparatus for printing a desired image on a printing medium, by construction from individual marks formed in a pixel column-and-row array. This apparatus includes a scanning printhead that makes multiple passes across the printing medium.
It also includes some means for establishing a dither mask having a dimension, and some means for establishing a printmask with a corresponding dimension. For this second aspect, however, it is not necessary to impose a nonmultiple/nonfactor constraint such as used in the first facet of the invention.
Instead this aspect of the invention includes some means for establishing an offset that is smaller than at least one of the dither-mask and printmask dimensionsxe2x80x94and also some means for indexing one of the dither mask and printmask by that offset, between passes of the scanning printhead. In other words, either the dither mask or the printmask is internally stepped or shifted. It is within the scope of the invention to index bothxe2x80x94but preferably not in such a way as to keep them consistently registered, since an object of the invention is to minimize consistent registration. Although xe2x80x9cindexingxe2x80x9d does not itself entail moving the mask envelope relative to the printing medium or relative to the printhead, naturally the mask envelope may also be displaced in a conventional fashion between printhead scans.)
In addition, this aspect of the invention includes some means for using the dither mask to render the image and using the printmask in controlling the printhead to print the rendered image.
This second aspect of the invention may be recognized as using essentially the same principle as the last-mentioned preference described above for the first aspect. This offset system can thus be used for one or both orthogonal dimensions in a rectangular-pixel-grid printer.
The foregoing may constitute a description or definition of the second facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention too significantly mitigates the difficulties left unresolved in the art.
In particular, this aspect of the invention is an unusually powerful tool for obtaining the same advantages provided by the first aspectxe2x80x94but when the nonmultiple/nonfactor constraint is unavailable or uneconomic, or for some other reason undesirable.
Although this second aspect of the invention in its broad form thus represents a significant advance in the art, it is preferably practiced in conjunction with certain other features or characteristics that further enhance enjoyment of overall benefits.
For example, this aspect of the invention is particularly useful if the dither-mask establishing means and the printmask-establishing means include one or more preprogrammed circuits or algorithms for implementing the dither mask and printmask. Here it is to be assumed that the printmask corresponding dimension is an integral multiple or an integral factor of the dither-mask dimension, and furthermore that the preprogrammed circuit is incapable of effectuating small changes in either of those dimensionsxe2x80x94or the preprogrammed algorithm is incapable of effectuating such changes efficiently.
In such cases it is typically very undesirable to reprogram the entire circuit or algorithm to introduce a major new strategy (such as the first aspect of the present invention), because of the associated costs and time required to thoroughly debug the revised code. These benefits are particularly powerful in the case of a preprogrammed application-specific integrated circuit (ASIC), because redesign of an ASIC is so monumentally expensive, time consuming and risky.
The second aspect of the invention can avoid all such costs, delays and risk if the system simply permits independent access to the contents of the printmask or dither mask in each pass. Such access is available in some systems, without revision of the algorithmic code, for the very reason that mask variations have often become desirable after completion of the code.
Thus the second aspect of the invention is particularly effective if the preprogrammed circuit is incapable of effectuating any change in either of the dimensions. This is the characteristic most likely to be present if the preprogrammed circuit is an ASIC.
For the second main aspect of the invention, as for the first, the question arises how to construct the apparatus if the apparatus prints in plural colors. Preferably the several components discussed here are replicated for each of the colors respectively.
In preferred embodiments of a third of its basic aspects or facets, the invention is a method of printing desired images on a printing medium The method operates by constructing the images from individual marks formed in a pixel column-and-row array by a scanning multiple-nozzle pen, which operates in conjunction with a printing-medium advance mechanism.
The method includes the step of establishing and operating a rendition stage that includes using a dither mask which has a dimension. It also includes the step of establishing and operating a printmasking stage that includes using a printmask. The printmask has a dimension which is an integral multiple or an integral factor of the dither-mask dimension.
The dither mask and printmask dimensions have an interaction that establishes a spatial periodicity which is well within a range to which human vision is sensitive. In addition, the method includes the step of extending the spatial periodicity of the interaction between the dimensionsxe2x80x94specifically, extending the periodicity to a value well outside that sensitive range.
The foregoing may represent a description of definition of the third aspect or facet of my invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention has generally the same advantages as those noted above for the second facet. This third aspect, however, is not necessarily limited to the particular small-offset methodology.
Although this method even as thus broadly couched serves an excellent purpose, nevertheless preferably it is performed with certain additional features or characteristics. For instance, preferably the extending step includes causing at least one of the mask dimensions to be effectively larger, in regard to its interaction with the other of the mask dimensions.
Further along these lines, still more preferably the causing step comprises effectively enlarging said at least one of the mask dimensions by a factor of at least two. A particularly strong preference is that the factor be at least eight.
Another preference is that the causing step include indexing the xe2x80x9cat least onexe2x80x9d of the masks by an offset that is less than half of that mask dimension. In this case the factor is equal to the ratio of (1) the at least one of the mask dimensions and (2) the offset.
A further specific desirable characteristicxe2x80x94for this indexing form of the third aspect of the inventionxe2x80x94is that the causing step cause the spatial periodicity to be equal to the minimum common multiple of (1) that same one of the mask dimensions, as thus effectively enlarged, and (2) the other of the mask dimensions.
Once again for this third facet of the invention, a further consideration comes into play if the images are to be printed in plural colors. Preferably the method is applied independently for each of the colors respectively.